Steroidal olefins, electrophilic addition

Oxidation of the steroidal olefin (XXVII) with thallium(III) acetate gives mainly the allylic acetates (XXXI)-(XXXIII) (Scheme 15), again indicating that trans oxythallation is the preferred reaction course (19). Addition of the electrophile takes place from the less-hindered a-side of the molecule to give the thallinium ion (XXVIII), which by loss of a proton from C-4 would give the alkylthallium diacetate (XXIX). Decomposition of this intermediate by a Type 5 process is probably favorable, as it leads to the resonance-stabilized allylic carbonium ion (XXX), from which the observed products can be derived. Evidence in support of the decomposition process shown in Scheme 15 has been obtained from a study of the exchange reaction between frawr-crotylmercuric acetate and thallium(III) acetate in acetic acid (Scheme 16) (142). 

Electrophilic Addition.—rrans-Diequatorial diols are not readily prepared from olefins in the steroid series. The ring-opening of epoxides generally gives trans-diaxial products, whereas acyloxonium ions (65), formed during the Prevost... 

Electrophilic additions of other reagents e.g. HOBr, INCO) to these olefins are quite normal in giving the diaxial products in high yield. The order in which the components of these molecules attack the steroid (1+ or Br+ first, followed by the anion) provides a most useful synthetic route to epoxides or ethyleneimines of the -configuration (p. 117), for the preferential formation of a halonium ion of a-configuration directs the 0- or iV species to the / -face of the molecule. It has already been noted that direct epoxidation of a steroid olefin usually gives the a-epoxide. 

Electrophilic Addition.— The olefinic bond in an ergost-22-ene undergoes stereospecific and regiospecific addition reactions, controlled by its dissymmetric environment. The preferred conformation, revealed by A-ray analysis, is represented by (77). Frontal hindrance by the steroid nucleus, and the 24-methyl... 

Electrophilic Addition.— The use of electrophilic fluorinating agents has been reviewed. A review on organoselenium chemistry includes data on the addition of PhSeBr to steroidal olefins. An improved procedure for chloro-sulphenylation was exemplified by treatment of 5a-cholest-2-ene with thio-phenol and iV-chlorosuccinimide. The resultant adduct (35) was readily converted into the unsaturated aryl sulphone (36). Bromine fluoride is conveniently... 

Iodine azide, on the other hand, forms pure adducts with A -, A - and A -steroids by a mechanism analogous to that proposed for iodine isocyanate additions. Reduction of such adducts can lead to aziridines. However, most reducing agents effect elimination of the elements of iodine azide from the /mwj -diaxial adducts of the A - and A -olefins rather than reduction of the azide function to the iodo amine. Thus, this sequence appears to be of little value for the synthesis of A-, B- or C-ring aziridines. It is worthy to note that based on experience with nonsteroidal systems the application of electrophilic reducing agents such as diborane or lithium aluminum hydride-aluminum chloride may yet prove effective for the desired reduction. Lithium aluminum hydride accomplishes aziridine formation from the A -adducts, Le., 16 -azido-17a-iodoandrostanes (97) in a one-step reaction. The scope of this addition has been considerably enhanced by the recent... 

The Johnson group began by examing the solvolysis of 5-hexenyl nosylate (65) in formic acid. This reaction gave only small amounts of cyclohexanol and cyclohexene, but illustrated that that cyclization was possible. Solvolysis of diene 66 gave cyclohexanol 67 as the major product. This product results from addition of the electrophilic carbon (Ci) and oxygen nucleophile (formate) across the fm r-olefin. Decalins 68-70 were produced in low yields, but it was notable that only fm -fused decalins were produced. This observation was consistent with an addition of electrophilic carbon (Ci) and nucleophilic carbon (Cio) across the af-olefin (see B in Steroids-12). 

Finally, solvolysis of diene 71 gave cyclohexanol 72 and m-decalins 73 as the most notable products (Steroids-14). Once again, the stereochemistry of these products suggest that at ft-addition of electrophile and nucleophile across the r -olefin was dictating the stereochemical course of cyclization reactions. This experiment also showed that the solvolyses of 66 and 71 do not pass through a common monocyclic intermediate. 

---